Genetics Using Punnett Squares

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Constance Golden
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1 Genetics Using Punnett Squaes Ealy Genetics The study of genetics began with obsevations made by Gego Mendel. Afte noticing that the flowes his pea plants wee eithe violet o white, Mendel began to study the segegation of heitable taits. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants. Remembe that Mendel woked almost 150 yeas ago when nobody knew about genes o even the stuctues (chomosomes) that cay genes. 1

Taits ae chaacteistics you inheit fom you paents.

2 Hee ae some taits obseved by Mendel: Lets conside a single gene A gene caies infomation that detemines you taits. Taits ae chaacteistics you inheit fom you paents. Genes ae located in chomosomes. Chomosomes come in pais and thee ae thousands, of genes in one chomosome. 2

3 Continued In humans, a cell s nucleus contains 46 individual chomosomes o 23 pais of chomosomes. Half of the chomosomes come fom one paent and half come fom the othe paent. Hee is the detailed stuctue of a chomosome Definitions Allele- discete vesion of the same gene Genotype- the genes of an oganism fo one specific tait Phenotype- the physical appeaance of a tait in an oganism 3

4 Definitions Dominant tait efes to a genetic featue that hides the ecessive tait in the phenotype of an individual. The tem "ecessive descibes a tait that is coveed ove (o dominated) by anothe fom of that tait and seems to disappea. Homozygous= two alleles that ae the same fo a tait (Pue) Heteozygous= two diffeent alleles fo a tait (Hybid) Pactice We use two lettes to epesent the genotype. A capital lette epesents the dominant fom of a gene (allele) and a lowecase lette is the abbeviation fo the ecessive fom of the gene (allele). Example below: P=dominant puple and p= ecessive white The phenotype fo this flowe is violet while its genotype (if homozygous) is PP. The phenotype fo this flowe is white while its genotype is pp (to be white the flowe must have two of the ecessive copies of the allele). 4

5 Punnett Squaes The Punnett squae is the standad way of woking out what the possible offsping of two paents will be. It is a helpful tool to show allelic combinations and pedict offsping atios. Befoe we go futhe lets eview how to set up a Punnett Squae We begin by constucting a gid of two pependicula lines. 5

6 Next, put the genotype of one paent acoss the top and the othe along the left side. Fo this example lets conside a genotype of BB cossed with bb. B B b b Notice only one lette goes above each box It does not matte which paent s genotype goes on eithe side. Next, fill in the boxes by copying the column and ow head-lettes down and acoss into the empty spaces. b B B b B B b b B b B b 6

7 Punnett Squaes Now that we have leaned the basics of genetics lets walk though some examples using Punnett Squaes. W w W w W W W w W w w w Lets say: Usually wite the capital lette fist W- dominant white w- ecessive violet Paents in this coss ae heteozygous (Ww). Note: Make sue I can tell you capital lettes fom lowecase lettes. What pecentage of the offsping will have violet flowes? ANSWER: 25% (homozygous ecessive) 7

8 Red hai (R) is dominant ove blond hai (). Make a coss between a heteozygous ed head and a blond. R What pecentage of the offsping will have ed hai? 50% Let s ty some moe In pea plants, tall pea plants (T) ae dominant ove shot pea plants (t). Constuct a Punnett Squae fo a heteozygous tall pea plant and a shot pea plant. t t T Tt Tt t tt tt What ae the pecentage of phenotypes? 50% tall 50% shot 8

Black eyes (R) is dominant ove ed eyes () in ats. Make a coss between a homozygous at with black eyes and a at with ed eyes. R R What is the possibility of a ed eye off spings?

9 Black eyes (R) is dominant ove ed eyes () in ats. Make a coss between a homozygous at with black eyes and a at with ed eyes. R R What is the possibility of a ed eye off spings? 0% Refeences actice.ppt#1 be/punnett.htm type.jpg 9

Gregor Johann Mendel ( ) Responsible for the Laws governing Inheritance of Traits

Gregor Johann Mendel ( ) Responsible for the Laws governing Inheritance of Traits Gego Johann Mendel (822-884) esponsible fo the Laws govening Inheitance of Taits Austian monk Studied the inheitance of taits in pea plants Developed the laws of inheitance Mendel's wok was not ecognized